Stormwater Management

Overview

All stormwater within the city limits drains into the Cibolo Creek, which drains into the San Antonio River. Much of the City of Boerne’s drainage issues have been around for decades. Many of our city streets were developed before current flood prevention techniques were adopted. Because the city does not have an extensive underground stormwater system, our streets are designed to carry runoff within the curb and right-of-way. Which means it is common for water to run along curbs and outside lanes of traffic while it is raining.

Boerne averages 34 inches of rain per year, but as we know, the level of variability is extreme. Since 1991, the city has recorded six years with more than 50 inches of rain and seven years with less than 25 inches of rain, with 1999, 2008, and 2011 recording less than 20 inches of rain for the entire year. September is our rainiest month of the year, followed by May. Both average more than three inches of rain. According to the Federal Emergency Management Agency, to date there have been no 100-year floods in Boerne. In other words, the chances that the creek will flow at the “100-year” flood stage in any given year is one percent.

Previous significant flood events in and around Boerne:

• September 1967: 17 inches in upper watershed, 4.47 inches in Boerne (before the dam creating Boerne Lake was built)
• June 1997: 18 inches in 30 hours in upper watershed, 8.15 inches in Boerne
• August 2001: 6.89 inches of rain in 24 hours
• July 2002: 30.9 inches of rain in six days
• May 2015: 9 inches in 24 hours, more than 20 inches for the entire month
• September 2018: Rained almost every day with the city recording 16.68 inches for the month.

Flood Control

According to FEMA, the 1964 flood is Boerne’s most damaging rainfall event and remains the flood of record.

Based on FEMA's guidelines, there is a two percent chance a storm of that magnitude can occur annually.

The flood caused the highest water rate in the creek’s recorded history at 36,400 cubic feet per second. Previously, this would have been classified as a “50-year” flood.

That historic storm led to the planning and development of ways to prevent future catastrophic flooding, which ultimately created four Soil Conservation Service sites – water retention dams – north and west of Boerne to serve as flood control. In January 1972, the Texas Water Commission granted permission to the City of Boerne to construct and maintain a dam on Cibolo Creek for municipal purposes to aid in flood control. Boerne Lake is the largest of the four SCS’s constructed after the flood of 1964. More than seven years later, the certificate was amended so the city could utilize water from the reservoir for domestic purposes, and in 2005 City Lake Park was developed.

While the uses of Boerne Lake have increased over the years, it was first and foremost a flood control structure. In total, the four SCS’s hold approximately 21,800 acres of watershed. In the City of Boerne, approximately 32,000 acres of watershed ultimately funnels into River Road Park in downtown Boerne. Of that, 24,400 acres of watershed occur outside of city regulations.

Past Drainage Master Plans

• 1968: Watershed Work Plan – resulted in the building of four water control dams
• 1991: Drainage Master Plan
• 2002: Updated Drainage Master Plan
  • Updated subdivision ordinance
  • Proposed creating a stormwater utility (passed by City Council in 2019). As of 2021, the stormwater utility is billing customers within the city limits $50,000/month. This money can only be used to water and drainage within the city limits.
  • Develop model for Currey Creek and No-Name Creek
  • Prepare plan for improvements, as well as finance and implement improvement plans
• 2021: Updated Drainage Master Plan
  • Citizens Flood Map: allowed residents to pinpoint trouble spots in need of upgrades
  • City staff is thinking of creative and adaptive ways to update flood prone areas that fit with the Hill Country esthetic and residents' desires.

Development Code Changes

• Subdivision Ordinance – February 2020
  • Created Drainageway Protection Zones and Local floodplain requirements to include areas not included in the federal flood maps. For example, previous FEMA floodplain maps stopped at Adler Road, the updated ordinance now includes watershed north of that area.
  • Provided more detail on Low Impact Development requirements
• Flood Damage Prevention Ordinance – August 2020
  • Greater restrictions on residential within floodplains
  • Finished Floor (FF) height changes
  • Net storage volume requirements, which means if a development changes the conditions of a water way, the developer must accommodate for that change so there is a net-neutral impact.
• Unified Development Code – July 2021
  • Changes to residential cross property drainage requirements
  • Detailed requirements for residential mass grading
  • Further detail on detention pond design
  • Groundwater recharge feature protection
  • Enhanced erosion protection requirements
    • City is not a regulated Municipal Separate Storm Sewer System (MS4) therefore erosion control enforcement is by Texas Commission for Environmental Quality region office. According to the TCEQ, an urbanized area is a densely settle core of census tracts or census blocks that have a population of 50,000 or more. It is a calculation used by the U.S. Census to determine the geographic boundaries of the most heavily developed and dense urban areas.
    • Phase II (small) MS4’s are established for areas located within Urbanized Area (UA) and outside the UA but designated by TCEQ.